Systematic changes in motor cortex cell activity with arm posture during directional isometric force generation.

We report here the activity of 96 cells in primate primary motor cortex (MI) during exertion of isometric forces at the hand in constant spatial directions, while the hand was at five to nine different spatial locations on a plane. The discharge of nearly all cells varied significantly with both hand location and the direction of isometric force before and during force-ramp generation as well as during static force-hold. In addition, nearly all cells displayed changes in the variation of their activity with force direction at different hand locations. This change in relationship was often expressed in part as a change in the cell's directional tuning at different hand locations. Cell directional tuning tended to shift systematically with hand location even though the direction of static force output at the hand remained constant. These directional effects were less pronounced before the onset of force output than after force onset. Cells also often showed planar modulations of discharge level with hand location. Sixteen proximal arm muscles showed similar effects, reflecting how hand location-dependent biomechanical factors altered their task-related activity. These findings indicate that MI single-cell activity does not covary exclusively with the level and direction of net force output at the hand and provides further evidence that MI contributes to the transformation between extrinsic and intrinsic representations of motor output during isometric force production.